Atomic Layer Deposition of Silica on Carbon Nanotubes

Abstract

Low-temperature\nozone-assisted atomic layer deposition (ALD) of\nSiO₂ with four silane derivatives (3-aminopropyl)­triethoxysilane\n(APTES), bis­(diethylamino)­silane (BDEAS), diphenylaminosilane (DPAS),\nand triethylsilane on carbon nanotubes (CNTs) leads to the one step\nformation of SiO₂ nanotubes. In the process, CNTs act as\ntemplates and are removed during the ongoing deposition. From transmission\nelectron microscopy images, the formation of a void between the CNTs\nsurface and the SiO₂ coating was observed, indicating an\nunexpected removal of carbon from the CNTs. This gap grows as the\nnumber of ALD cycles is increased, eventually leading to SiO₂ nanotubes almost free of carbon. ATR-IR and EELS spectra proved\nthe SiO₂ formation. Depending on the CNTs templates used\nin this process, different morphologies of one-dimensional SiO₂ nanostructures are obtained, including simple nanotubes,\nhollow wall nanotubes, tube-in-tube structures, and SiO₂ nanowires. The application of this process on vertically aligned\nCNTs (VACNTs) templates allows the formation of a perfect SiO₂ replica of the VACNTs. From experiments with different oxygen\nand silicon precursors, it is proposed that peroxides and oxygen-based\nradicals, which can be formed from the reaction of surface Si–H\nspecies with ozone, are the main reactive species leading to the unexpected\netching of carbon from the CNTs during silica ALD.